1. Field of the Invention
Embodiments of the present invention generally relate to a process and apparatus for the production of methylene diphenyl diisocyanate (MDI) isomer mixtures with low 2,2′-MDI isomer content and high 2,4′-MDI isomer content.
2. Description of the Related Art
Mixtures of methylene diphenyl diisocyanate (MDI) isomers are widely used in the preparation of polyurethane film composites and adhesives. During the preparation of film composites or adhesives, an MDI mixture reacts with a polyol (e.g., polyether polyol) to form polyurethane. Generally, an excess of MDI is used to ensure the completion of the reaction. However, non-reacted MDI can diffuse to the surface of the film composite or adhesive, where it may be hydrolyzed into a primary aromatic amine. This creates a problem in certain applications, such as product packaging in the food industry because there are strict requirements regulating the amount of primary aromatic amines in food packaging materials.
Conventional MDI processes produce three isomers, i.e., 4,4′-MDI, 2,4′-MDI, and 2,2′-MDI. Other products of conventional MDI processes are heavier molecular weight isocyanates, commonly called polymeric MDI (PMDI). The most reactive MDI isomer is 4,4′-MDI, and the least reactive is 2,2′-MDI. Thus, among MDI isomers, 2,2′-MDI requires the longest time for conversion, and the overall reactivity of an MDI isomer mixture increases as the 2,2′-MDI isomer content is reduced. However, the ratio of 2,2′-MDI and 2,4′-MDI to 4,4′-MDI also controls the viscosity of the prepolymer. That is, prepolymer viscosity decreases as the ratio of 2,2′-MDI and 2,4′-MDI to 4,4′-MDI increases. Thus, mixtures with a high amount of 2,4′-MDI and a low amount of 2,2′-MDI are used in applications requiring low viscosity prepolymers, particularly in the food industry.
Therefore, as the demand for low viscosity prepolymers increases, such as for food applications, there is a need for improved methods and apparatus for producing MDI isomer mixtures with low 2,2′-MDI content and high 2,4′-MDI content.